1. Field of the Invention
The present invention relates to a method and apparatus for imaging blood vessels employing magnetic resonance and, more specifically, to an imaging method and apparatus which permit visualization of only arteries, only veins or both the arteries and veins.
2. Description of the Related Art
A description will be made of a conventional method of imaging blood vessels, which is generally referred to as magnetic resonance (MR) angiography. As shown in FIG. 1, let us imagine a slice P which is perpendicular to the direction in which a blood vessel V runs. A radio frequency (RF) pulse-is applied to the slice P to excite it. Then, the entire slice is excited. In this case, fresh blood which has not been subjected to the RF pulse flows into the blood vessel V in the slice P. For this reason an MR signal at a high level will be generated from the blood vessel V. Since, in this case, fresh blood flows into the vessel with each repetition of the RF pulse, a phenomenon will occur in which a high-level MR signal is produced each time an RF pulse is applied. This is generally referred to as the time-of-flight phenomenon. Also, there is a phenomenon in which the phase of the MR signal varies because of the flow of blood. This is generally referred to as the phase modulation phenomenon. There is a method which employs these phenomena to obtain MR angiograms.
Besides the above methods, there is the following method. First, in this method, a single-slice pulse sequence whose pulse repetition period Tr is short is adopted. In executing this pulse sequence a plurality of slices are excited in sequence for visualization of plural regions. During this process a saturation phenomenon due to the short repetition period and the time-of-flight phenomenon act, whereby the amount of generation of MR signals from static portions becomes small and eventually high-level MR signals are obtained selectively from flowing blood, so that MR angiograms are obtained.
Every one of the above-described conventional methods permits presentation of desirable angiograms for blood flow (blood vessels) in one direction. However, in the case where an artery and a vein are mixed, blood flow in one direction and blood flow in the other direction cannot be separated from each other. For this reason the artery and the vein are presented together. Thus, they are hardly distinguishable from each other and the blood flow is also difficult to understand properly.